Is an Unvented Gas Fireplace More Efficient Than a Condensing Furnace?

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One of the primary benefits of an unvented gas fireplace is that you don't lose any heat up the flue. That's because there isn't a flue, of course. That's one of the main reasons supporters of unvented gas fireplaces use to sell a product that's banned in California and Canada. "Hey, 100% beats 95%!" (The potential problems with indoor air quality and moisture, however, outweigh any benefits so don't run out and buy one just yet. Or ever. Don't fall for the hype.)

But let's examine the facts. Yes, even the highest efficiency condensing furnaces still lose some heat in the exhaust gases that go up the flue. And 100% efficient really does sound better than 95%, although both are good. So does that mean unvented fireplaces are more efficient than condensing furnaces?

Heating efficiency numbers

Let's take a look at the efficiency values for these two heating appliances. Before we get to them, though, let's talk about the standard furnace: the 80 AFUE induced draft furnace. AFUE stands for annual fuel utilization efficiency and is the percent efficiency, averaged over a year. We see a lot of that type here in the Southeast. When the gas gets burned in one of these furnaces, 80% of the energy in the gas gets transferred to the air moving through the furnace, which then goes into the conditioned space. 20% of the heat goes up the flue.

A high-efficiency, condensing furnace does much better. They start at about 92%, although most condensing furnaces these days are 95% and higher. We did a quick search on the AHRI Directory this week and the highest efficiency model we found comes in at 97.7%.

When you look for the efficiency of unvented gas fireplaces, you see claims for 99% or 99.9%. (For purposes of this article, let's just say it's 99.9%.) That number is so high because, as I said at the beginning, none of the heat is escaping up the flue.

Percentage of what?

Based on those numbers, it looks like the unvented gas fireplace (also called euphemistically ventless or vent-free) is the winner. Not many people† would dispute that 99 is a bigger number than 97.7. But there's a hidden problem here. We're comparing apples to oranges when we compare 99% to 97.7%.

Here's why. Both the unvented gas fireplace and the condensing furnace have a combustion efficiency of nearly 100%. That means almost every one of the methane molecules combines with two molecules of oxygen, producing one molecule of carbon dioxide, two molecules of water vapor, and some heat.

The unvented gas fireplace keeps all of that heat in the house, which is how they claim 99.9% efficiency. The only loss is from the little bit of incomplete combustion that happens. But there's something missing here.

The missing link

In that combustion reaction, there are three products: carbon dioxide, water vapor, and heat. What's lacking in the efficiency rating for the unvented gas appliance is the latent heat of the water vapor. And that's where the name of the condensing furnace comes in. That water vapor has a lot of energy in it. When it condenses, it gives up heat. In a condensing furnace, there's a secondary heat exchanger whose purpose is to condense the water vapor and capture that heat.

So, let's introduce a couple of new terms here. When we talk about natural gas as a heating fuel, it has two different heating values. The Lower Heating Value (LHV) is the amount of heat you get from the combustion and from bringing all the combustion products back to the original temperature without condensing the water vapor. That's what's being used in the 99.9% efficiency rating for unvented gas fireplaces.

The Higher Heating Value (HHV) has three components: (1) the heat given off during combustion, (2) the heat regained by bringing the combustion products back to the original temperature, and (3) the heat that comes from condensing the water vapor. And that's what the efficiency of condensing (and standard 80 AFUE) furnaces is based on. (The Wikipedia page on heat of combustion has a good explanation of lower and higher heating value.)

How much does of a difference does this make? If the unvented gas fireplace manufacturers reported efficiency relative to the Higher Heating Value, they would come in at about 91%. So a condensing furnace at 95-98% efficiency would get you get 4-7% more heat from the same amount of fuel than an unvented gas fireplace.

Don't skip the comments

I'm a very fortunate blogger. I find interesting things to write about in the world of building science and have attracted some very smart readers. Some of them post comments that add a lot to the discussion at hand or raise new topics to discuss. That's the case here.

When I wrote about the first law of thermodynamics in November 2017, RoyC posed the topic of this article as a little quiz in the comments. I thought I knew the answer but turned out to be wrong. I had assumed that a 99.9% efficient unvented gas fireplace would be more efficient than a 95% condensing furnace. But RoyC is a smart guy with a heck of a lot of engineering experience and knowledge, and he helped me see the bigger picture here.

So, when you read articles here in the Energy Vanguard Blog, don't skip the comments. Not only could you learn something new, but you might also get a preview of a future article.

Comments

I understand the difference between lower and higher heating value and that by condensing the water vapor product of combustion a condensing gas furnace may transfer ~5% more sensible heat to the home.

But that doesn't tell the whole comfort story - many homeowners invest considerable energy (~1000 Btu per pint or pound) in evaporating liquid water in an effort to humidify their homes during the heating season.

An unvented gas heater adds water vapor to the indoor air, arguably desirable in that it could offset some or all of the energy needed to humidify a home in winter.

Considering this from another angle - enthalpy- the total heat content of the air / water vapor mixture in a home. If one counts the heat energy smbodied in the water vapor added to a home's indoor air by an unvented heater, then the unvented heater gains the 3-5% edge over the condensing furnace.

All that said, I would never never ever recommend or install an unvented gas appliance...if just a few of the carbon atoms combine with just one rather than two oxygen molecules (incomplete combustion) the result is carbon monoxide - bad news in even small quantities.

Lastly don't forget the Methanethiol, AKA Methyl Mercaptan or just Mercaptan...supposedly harmless but stinky stuff added to fuel gases as an odorant for safety...who wants that stuff in their indoor air?

The best solution for IAQ and safety is to neither heat nor cook (indoors, anyway) with any gas...use heat pumps and let the pros, i.e power plant operators, manage the combustion process somewhere else. Or, better yet, consume electricity generated from renewable sources.

While it is true that there are people who humdidify their buildings, this should become a much smaller part of the population. The only reason you would humidify is if you are in a cold climate and 1. your house is way too leaky or 2. your house is way over ventilated or 3. you want the humidity way too high and enjoy mold and moisture problems.
All three of those reasons should be attacked first before adding a humidifier.
However, the IAQ risks of unvented gas fireplaces are so high, it is not worth even trying to take advantage of the humidity given off even when this is a benefit. In modern home excess humidity is much more of a problem.
Thankfully, Canada and numerous states have banned the sale of these devices.

I appreciate this and found it interesting. It's also sad that any argument besides "efficiency doesn't matter when you're dead" has to be made.

Also, I would like point out that last couple of unvented heaters I have encountered were not achieving 100% combustion efficiency. Most of the time I see these unvented heaters in situations where they get no maintenance whatsoever. The amount CO they often produce, and in some cases the soot on the ceiling above them, is evidence of this.

Curt has a good point in that if you have a leaky house in a dry winter climate where you might run a humidifier, you can take advantage of the water vapor in the flue gas of an unvented heater. However, if you have a tight house that does not need additional humidification in the winter, you can still recover some of the latent heat of the water vapor from an unvented heater by letting it condense on the windows, but maybe that is not a good idea.

My biggest concern is unvented gas logs. Some people might think that they are good in new tight houses since you don't have to have another flue leaking air to the outside and that they are "99.9% efficient". However, you could end up with humidity problems if the house is sufficiently tight. Also, there have been some studies showing that NO2 generation might be a long-term health concern.

I agree that artificial humidification is a band-aid masking other problems. I'm an HVAC contractor in Florida, and despite the lost potential profits, I treat central DE-humidifiers as an expensive band-aid to be avoided if at all possible.

I explain the infiltration issue to homeowners in part by joking (sort of) that while I expect to succeed at cooling and dehumidifying their home but I can't be expected to take on their yard and the rest of the neighborhood.

Allison:
not sure why you seem to be promoting these devices, given the IEQ & health hazards, carbon emissions, and healthy alternatives. Please note that unvented gas fireplaces are illegal in CA. Last time I checked (years ago), we were the only state that did not allow them to be sold. The gas utilities and Wx firms will unhook them before doing any weatherization.

Technically, these devices could be sold in CA if the Dept. of Health set a health-based emission standard, but the industry decided not to pursue that path after modeling showing IAQ problems.

Alison:
My reaction was because 1) I was inititally surprised that you were even bothering to address the EE issues of unvented gas heaters. 2) I did see your mention of IEQ problems, but the "just yet" opened the door to doubts and muddied the message that such devices should just be removed or replaced with a vented device.

I am glad you had stronger recommendations in your previous posting about the liability risks of such devices (I did check that out later). IMHO, this should have been highlighted in the first line of your latest post, followed by a discussion of any new info of avoiding such health hazards.

Good point, Tom. I guess I was blinded by the curse of knowledge. Since I've stated my position so strongly in so many other articles, I figured everyone reading would know that. Thanks for pointing that out.

But let me ask you something. You said the "just yet" weakened the message, and I agree. That's why on my second or third reading I added the "Or ever." Did that not make it strong enough?

re: "...just yet. Or ever." It's still a bit ambiguous, unless the reader goes to your previous article re: liability. And the title and preceding sentences are about EE (without context that another commenter mentioned re: industry claims).

Your statement is not strong enough. The quick answer to all of this is: "Do NOT use these devices EVER. File under Stupid Human Tricks."

The more pertinent questions would be how many homes use these devices, and how to replace the devices with safe, low C products and to take additional measures to reduce heating demand. I imagine there are some good stories from folks who have done this.

Nice academic exercise, but when you back up and look at the house as a system, there's a lot more involved than (potentially) a 3 or 4% bump in combustion efficiency.

Aside from releasing bad stuff into your breathing air and possibly putting the structure at risk for mold & rot, there's the additional heat loss that occurs when a fireplace backs up to an exterior wall. That alone likely dwarfs flue losses from a high efficiency furnace.

Also, point heat sources (especially fireplaces) waste energy by overheating the immediate vicinity without a means to evenly distribute heat to other rooms.

Methinks those who would make the argument that unvented gas fireplaces are more efficient than a furnace are simply grasping at straws to rationalize a dumb product.

All good points, David. And of course, those of us in the world of building science always back up and look at the big picture, but many people get swayed by little things like 100% efficiency (compared to 95% or 80%). And yes, this is one of the two straws that they grasp at. The other is cost, since it's cheaper to install a fireplace without a flue than one with a flue.

I've seen unvented heaters mostly on Habitat critical repair inspections lately. Although anecdotal, I feel like I could make a argument that the legality of these disproportionately effects the health low income families. My theory being that they appear as a low cost, DIY heat source to people with limited funds.

The point of my original past comment and Allison expanding on it here is that the unvented heater guys (space heaters, gas logs, etc.) claim that their devices are highly efficient at heating and that is just not true. Their only advantage is low installed cost. There are several potential IAQ issues with them. They do have a good "safety" record which means that people don't drop over dead due to to carbon monoxide poisoning or oxygen depletion, partly because they are required to have an on-board oxygen sensor. However, that does not mean that there are not other potential long-term health effects, such as NO2 emissions.

Rumor has it that the ASHRAE 62.2 committee has received comments from the manufacturers about a change to the standard that would severely limit the use of these items. The rumor further says that their arguments were thoroughly considered and rejected based on science. I am not sure who this is up to now.

I'm no physicist and have no interest in defending unvented combustion heaters, but let's not forget the additional energy required to achieve that 95% AFUE in the condensing furnace, e.g.: electricity to mechanically force air over the heat exchanger(s) and through ducts to its final destination, the mechanical assistance needed to get those cooled exhaust gases outside the building envelope, and even some occasional energy to pump the condensed liquid into a drain. That electricity is more than likely harvested by relatively inefficient (~33%?) combustion at a power plant and transferred over a lossy grid for miles to the point of use. Shouldn't this also be factored into the equation? (And in case anyone wants to go there, I'm not looking to take this conversation beyond marginal operational cost, i.e. the energy required to manufacture, deliver, and install the appliance in the first place).

I see these every now and then here in Austin. The problem I always see is that you rely on ventless gas fireplaces being installed yet. On top of mostiure and iaq, you could have a potential of a big air infiltration leak if installed poorly.

The remark about low income struck a chord...as a just starting out college grad I leased an apartment in suburban Philly that combined a horribly inefficient envelope with baseboard electric resistance heat in a high cost electric utility territory.

There is one worse alternative than an unvented gas appliance, and I chose it...an unvented portable kerosene heater! Somehow I managed to neither die of the fumes nor burn down the building. Since then I've developed the impression that portable kerosene heaters are used by some of the lowest income, primarily rural residents.

Curt: Interesting that you brought up portable kerosene heaters. Their sales surged in the 80's when I was a grad student and my adviser got funding to test them for IAQ impacts in our research house. They burn quite "clean", especially if you have clean fuel, but we still showed that you pretty much need to open a window or have a real leaky house to be safe. The sales pretty much died off by the end of the 80's, but I see that they are still sold in the U.S. They also sell a special high-grade kerosene for those heaters that costs about $8/gallon. Doing some rough calculations, $8/gallon kerosene can provide heat at about $60 per million Btu, whereas $0.15/kWh electricity can provide resistance heat at about $44 per million Btu. You can buy kerosene for much less than $8/gallon, but then your IAQ risk probably goes way up.

You know, I seem to recall that unvented gas heaters weren't really 99%, but were actually about 92%. But for the life of me, I couldn't figure out how or why. This article nailed down everything. The how, the why, the "real" way to rate heating appliances. We don't even have to get into the difference in throw and circulation that a forced air system gives off compared to a stationary unvented gas fireplace or heater box.

Thank you Allison for bringing this up again. Until every state makes them illegal purchasers and home inspectors will not have to acknowledge the problem of unhealthy hearths. There should be a coordinated effort in every state to change the building code by credible associations that test, design and manufacture vented gas fireplaces. Until there change keep advocating.

Blake Basically the manufacturers of vented products are members of AHRI as are the manufacturers of the house vented heaters. Manufacturers are not likely to get into this fight. It is up to ASHRAE and all local folks who believe in indoor air quality.

Brilliant post! I never really thought that the energy from condensing the water would remain in the conditioned area.

Regarding the "we need to ban these" bandwagon, I think a step back should be taken. Gas logs are primarily used for occasional ambiance, not a primary heating source. I have one, and I can say with great certainty that I've done more unvented burning of natural gas via my stove/oven than I have with the ventless gas log. If you're worried about poor people misusing heat sources, then you should focus on gas/propane ovens. Growing up in poor, rural Michigan, I can tell you that's the "go-to" hillbilly temporary furnace.

Yea, if it is a decorative gas log, it may be used only occasionally, but some of these have pretty high capacity. My bigger concern is the use of unvented gas heaters which are only used for heating, not ambiance. I still wouldn't have either in my house.

Whether one calls it a decorative gas log, or a heater, there is nothing that keeps people from running them for long periods. I do not argue that unvented gas ovens are good, or that gas cook tops are good for the indoor air quality and our health. Gas ovens used to be vented by the way. My indoor air quality monitor detects when we use the electric oven and when we use the gas cooktop with the power venting always on. For years people have been testing ovens and cooktops for CO and know that there are numerous incidents of problem emissions. These same people have not had the equipment to measure NO2. If they did, we would see the real life teeter-totter between NO2 and CO.
One of the buried items from the NCAT Critical Needs Weatherization Research Project was:
"In a 1987 Pennsylvania study of low income weatherization, N02 concentrations were measured in the fall (prior to air sealing) and mid-winter (after air sealing). There were three homes in Pittsburgh that had electric ranges. These three homes averaged 0.013 ppm N02 in both the fall and mid-winter. On the other hand, the 12 homes in Pittsburgh equipped with gas ranges had an average N02 concentration of 0.046 ppm prior to air sealing and 0.071 ppm after air sealing. A number of these units exceeded the 0.08 ppm World Health Organization for 8 hour exposure. The average measured mid-winter N02 concentrations in the Philadelphia gas range homes was 0.09 ppm (above the WHO standard)". (GRI1987)